In a typical X-ray laser, the laser medium undergoes a suitable population inversion when light from a pumping laser is focused into one or more high-intensity focusing lines that are incident on a suitable target comprising the X-ray gain medium. Achieving a population inversion in the X-ray gain medium requires an input of highly intense energy in order to achieve the required ionizations in the gain medium. Several techniques are known for increasing the intensity of light from the pumping laser incident on the target to a sufficiently high level. One way is to focus the pumping-laser light as multiple focal lines on the target. The focal lines, each having a narrow width, are superimposed on the target. To such end, a lens with multiple focal lines can be used.
Conventional multiple-focal-line lenses have been manufactured at Quebec University in Canada and at the Shanghai Optical Precision Machine Research Institute in China. These lenses reportedly have been used in X-ray lasers. The lenses are configured as multiple cylindrical or convex lenses arranged vertically into several optical elements each capable of producing a line of pumping-laser light on the target. Exploiting a prism effect, the pumping laser light beam passes through each of the optical elements to "fire" simultaneously across all the focal lines. Such lenses can be cemented or non-cemented.
Unfortunately, splitting a cylindrical lens or a concave lens vertically or using a cemented configuration to form multiple focal lines does not ease the difficulty of making the width of the multiple focal lines sufficiently narrow to achieve the requisite high density of pumping radiation. That is, when a parallel pumping laser beam is directed into such a lens, the multiple-focal-line width cannot be made less than about 10 .mu.m; in fact, the line width is usually about 100 .mu.m or more. In the case of X-ray laser gain media that generate polyvalent ions when pumped, the wider the focal lines, the greater the required energy of each line (i.e., energy level is proportional to the line width). The minimum energy level required simply cannot be practicably reached with devices requiring a miniaturized X-ray source or X-ray laser. Furthermore, it has heretofore not be feasible to combine multiple split optical elements so as to produce multiple, narrow focal lines of pumping radiation with a high degree of accuracy.